1. Field of the Invention
The present invention relates to a rack-pinion type electrically powered steering apparatus and more particularly to an electrically powered steering apparatus provided with a device adapted to attenuate the driving current to the motor for power assistance in the vicinity of both ends of the rack when a steering wheel is operated.
2. Prior Art
An electrically powered steering apparatus of prior art is normally provided with a torque sensor adapted to respond to the steering load and a speed sensor adapted to respond to the speed of a vehicle. The electrically powered steering apparatus is further provided with an electronic control circuit for driving a motor for power assistance, which is adapted to input the detected signals from the torque sensor and the speed sensor and to output a pulse signal which has been pulse-width-modulated and amplified, the pulse signal having a steering load response characteristic and a speed response characteristic, so that when a steering wheel is operated the power assisting amount of time can be properly established.
In the conventional electrically powered steering apparatus as explained above, the driving current, which is applied from a battery installed in the vehicle, to the motor for power assistance under the control of the electronic control circuit is kept at an extremely small value when the vehicle is moving at a relatively high speed with little steering load, but is increased to a fairly large value when the vehicle is moving at a low speed or is at a stop. Consequently there have been certain problems in that, when the steering condition is continuously maintained at a location near the both ends of the rack in a steering mechanism, the power consumption caused by the electronic control circuit and the motor may increase to an extent that results in overdischarging of the battery installed in the vehicle and may also cause the windings of the motor to be overheated.